The researchers simulated a person’s cough by creating a burst of microscopic droplets
using a smoke generator and a manual pump... The water–glycerol droplets were 1–10
μm in diameter, similar to the smallest droplets expelled in a real cough, and were imaged
using a laser sheet.
With no mask, the aerosol jet extended an average of 8 feet from the head.
A handkerchief folded according to
instructions from the US Surgeon General reduced that
distance to 1 foot 3 inches.
A store-bought cone-style mask performed better, with the jet only extending 8 inches.
The best result came from a mask stitched out of two layers of quilting cotton (second figure), which permitted only a 2.5-inch jet.
A single-layer bandana with a higher thread count than the quilting cotton—85 rather than 70 threads per inch—performed much worse than any of the masks, with droplets spreading 3 feet 7 inches away. The result suggests that rather than a higher thread count, mask construction, including layering, may be more important for reducing droplet spread.
Although detailed quantitative studies are needed to fully characterize the wide range of face coverings now being used as personal protective equipment, the visualizations presented by Verma and coworkers can provide a quick assessment of a mask’s efficacy and help convey the importance of mask wearing. (S. Verma, M. Dhanak, J. Frankenfield,
Phys. Fluids 32, 061708, 2020.)
